Open Source Apps: Gateway Security Appliance

Like Untangle, the open source version of Endian allows you to turn an older PC into a gateway security appliance. It includes a firewall, anti-virus, anti-spam, Web content filtering, a VPN and more. Operating System: Linux

With Untangle, you can turn an old PC in to an appliance that protects your privacy and secures your network (or you can purchase a pre-configured appliance directly from the company). It's currently used by more than 1.7 million people at more than 30,000 organizations around the world. Operating System: Linux

Available as either a free open source download for up to five concurrent users or in an enterprise version for unlimited users, NetCop offers the same functions as Endian and Untangle and the commercial UTMs. However, it is not available as a pre-configured appliance. Operating System: Linux.

Open Source Apps: Genealogy

This Java-based genealogy app lets you view family information as a family tree, in a table, on a timeline, or by geographic location. It's appropriate both for people looking for a hobby and more serious historians. Operating System: OS Independent

If tracing your genealogy is your idea of a good time, you might enjoy this app which also has one of the most fun acronyms ever—Gramps is short for "Genealogical Research and Analysis Management Programming System." Not only is the tool itself helpful, but the site also include more than 1,000 pages of content about tracking your relatives. Operating System: Windows, Linux, OS X

Working on your family tree? PhpGedView makes it easy to collaborate online with the rest of your family to trace your genealogy. Note that you'll need your own Web server in order to set it up. Operating System: OS Independent

Open Source Apps: Geography/GPS

Another KDE app, Marble is a combination atlas/virtual globe that integrates with Wikipedia so that students can learn more about places that interest them. It also includes topographic maps, a satellite view, street maps, earth at night and temperature and precipitation maps, and many teachers find it useful in the classroom. Operating System: Windows, Linux, OS X

WorldWind was originally packaged to be very similar to Google Earth. The link here takes you to an SDK that allows you to incorporate WorldWind into your own apps. It also includes links to several interesting apps that use WorldWind. Operating System: OS Independent

Open Source Apps: Graphics Editors

This 3D modeling and rendering software isn't as full-featured as Maya (or Blender), but it's capabilities are robust enough for beginners and amateurs. The interface is fairly easy to use and it has good online documentation, including some tutorials. Operating System: OS Independent

If you need to outline the steps necessary for a project, create a system diagram or draw an org chart, Visio is for you. It supports common graphic file formats, as well as a space-saving custom XML format. Operating System: Windows, Linux, OS X

Short for "GNU Image Manipulation Program," GIMP is a professional-quality image manipulation program that's intuitive enough for amateurs to use. It includes a full suite of painting and image re-touching tools, layers and channels, sub-pixel sampling, quickmask, file format conversion tools, animation capabilities and much more. For the Windows version, you'll need to download Gimp-win. Operating System: Windows, Linux

Another tool for graphics professionals, Inkscape is a vector graphics drawing program with many advanced features. The Inkscape website also includes links to a library of open source clip art that you can use freely in your illustrations. Operating System: Windows, Linux, OS X

Similar to Photoshop and the open source graphics program Gimp, Pixelitor lets users perform advanced editing of photos and graphics. And because it's Java-based, it works on multiple platforms. Operating System: OS Independent.

Open Source Apps: Human Resource Management (HRM)

Open Applicant helps HR professionals and hiring managers sort through potential candidates to find the best applicant for a job. The application is free to download, but enterprise level support, customization, an SaaS version, and other services can be purchased from the company. Operating System: OS Independent

With more than 1 million users, Orange claims to be the "world’s most popular Open Source Human Resource Management Software." It tracks time and attendance, leave, recruiting performance, and employee information, and it's also available in an SaaS version. Operating System: Windows, Linux, Unix, OS X

While not as mature as Orange, Waypoint also tracks employee records, pay, performance, leave and other HR information. Also, like Orange, it's available in a hosted "On Demand" version. Operating System: OS Independent

Open Source Apps: Instant Messaging

Conceived as an MSN Messenger clone (before Microsoft changed the name to Windows Live Messenger), this app includes offline messaging, voice clips, photo support, custom emoticons and more. A large library of plug-ins and skins is also available on the site. Operating System: Windows, Linux, OS X

Small and fast, Miranda is very light on system resources. Like many of the other IM clients on our list, it supports multiple networks, including AIM, Facebook, ICQ, IRC, Yahoo and others. Operating System: Windows

Psi is an open-source IM client for the Jabber network, which includes GoogleTalk, LiveJournal, and a number of international networks. It’s completely customizable and supports about 20 different languages. Operating System: Windows, Linux, OS X

Open Source Apps: IT Inventory Management

This app creates a database that tracks all of the technical resources of your organization. It also includes some management functions that allows admins and help desk to staff track open jobs, respond to alerts, etc. Operating System: OS Independent

This "next generation" inventory tool helps you discover all the hardware and software in use on your network, which you can then track with a tool like GLPI (see below). It can also help you easily deploy scripts or software across your network. Operating System: OS Independent

Open Source Apps: Library

Koha humbly describes itself as "the most functionally advanced open source ILS on the market today." A variety of paid services, including hosting, support, implementation and consulting are also available on the site. Operating System: OS Independent

This automated library system includes online public access catalog (OPAC), circulation, cataloging, and staff administration functionality. It's not as full-featured as some of the other library apps, but it gets the job done, and the site has lots of documentation. Operating System: OS Independent

The second of the two most popular Linux desktop environments, KDE has a reputation for being "prettier" than some of the other desktop environments and feels very similar to Windows and OSX. The KDE community is one of the largest open source communities, so plenty of help is available for new (or experienced) users. Operating System: Linux

The "Lightweight X Desktop Environment" claims it has a "beautiful interface," but if truth be told, it's much more concerned about speed than looks. Because it uses so few computing resources, it's a good choice for netbooks and cloud-computing environments. Operating System: Linux

Another lightweight option, Xfce also offers very fast performance and doesn't make any claims about its beauty. It offers a modular design, so users can install just the components they choose in order to create a completely customized desktop experience. Operating System: Linux

Open Source Apps: Log File Monitoring and Analysis

The self-proclaimed "most popular logfile analyzer in the world," Analog quickly generates usage statistics for Web servers. It can be used in conjunction with Report Magic to create more attractive graphs. Note that this project has not been updated in a while, but it is still used to analyze traffic on many servers. Operating System: Windows, Linux, OS X

The "Basic Analysis and Security Engine," or BASE, use a Web-based interface to analyze alerts from Snort IDS. Features include role-based user authentication and Web-based setup. Operating System: OS Independent

The Snare project encompasses a number of different tools and agents, all of which assist in the filtering, collection and monitoring of server log files. Commercial support and the proprietary Snare Server are also available on the same site. Operating System: Windows, Linux, OS X, others

Like AWStats and Analyzer, Webalyzer analyzes the statistics from Web servers. By default, it creates yearly and monthly usage reports which can be viewed from any browser. Operating System: Windows, Linux, OS X

Open Source Apps: Logic/Debate

Similar to mind mapping software, Argumentative helps you create an "argument map" that outlines the premises of an argument in a visual way. It's particularly helpful for projects with a lot of writing – like creating a book, website, software documentation, etc. Operating System: Windows.

Open Source Apps: Mail Servers

Calling itself the "leader in true open source email and collaboration," Citadel features a unique "rooms" architecture that makes it versatile and flexible. It's available as a standard download, a VMware appliance or on a hosted basis. Operating System: Linux

This MTA was developed at the University of Cambridge and is still widely used in the UK. It's best for servers that are unlikely to have large volumes of mail because it doesn't have a central queue manager. Operating System: Linux, Unix

Originally developed by IBM Research, Postfix is a secure mail transfer agent that is very similar to Sendmail (see below). It's fairly fast and can handle a large volume of mail. Operating System: Linux, Unix, OS X, Solaris

Used by more than 1,300 corporations with more than 2 million mailboxes, Scalix offers an alternative to a Microsoft Exchange server. It offers e-mail and calendaring, with excellent Outlook support. It's available in a free community version or paid small business, enterprise or hosting versions. Operating System: Linux

While it's not as widely used as it once was, Sendmail continues to be a very popular mail transfer agent. Newer features include support for filters, authentication and external database look-ups. Operating System: Linux

Based on CentOS and Red Hat Enterprise Linux, SME Server offers file and printer sharing, a mail server, network firewall, automatic backup, remote access and more. Installation and basic configuration take only about 20 minutes, and there's quite a bit of documentation for the project available on the site's wiki. Operating System: Linux

The self-proclaimed "leader in open source e-mail and collaboration," Zimbra offers a mail server that provides an alternative to Exchange, as well as a desktop client that offers an alternative to Outlook. It's available in a variety of free and paid editions, including hosted versions and versions designed to run in a virtualized or private cloud environment. Operating System: Linux, Unix, OS X

The award-winning Dr. Geo helps both elementary (age 10 and up) and more advanced students learn basic geometric principles by interacting with geometric shapes. Check out the video on the website to see it in action. Operating System: Windows, Linux, OS X

This tool began as a simple calculator but has morphed into a powerful tool with many similar features as Mathematica (although it doesn't have Mathematica's full feature set). It has its own language (GEL), which can be used by researchers to create new functions. Operating System: Linux, OS X

GeoGebra combines tools for learning dynamic geometry, algebra and calculus. It offers an intuitive GUI and a video tutorial which make it a little more user friendly than some of the similar math apps. Operating System: OS Independent

If you're comfortable working from the command line, you can use gnuplot to create 2D and 3D graphs of mathematical functions. Extensive help is available on the website, and there's even a book on Gnuplot that you can buy. Operating System: Windows, Linux, Unix, OS X, and others

Why buy a handheld graphing calculator, when GraphCalc will do all the same things (and more) for free. According to the website, "GraphCalc can be your first, last, and only line of offense against the mathematics that threaten to push you over the brink of insanity. It slices, dices, shreds and purees functions that leave other calculators wondering what hit them." Operating System: Windows, Linux

This KDE app for geometry students and teacher makes it easy to draw and explore geometric shapes. It can also import files from Dr. Geo and Cabri. (Note that in order to use Kig on Windows, you'll have to download KDE for Windows.) Operating System: Windows, Linux

If you're geeky enough to know what a Mandelbulb is, you might enjoy this app which renders 3D fractals. More information about the app can also be found on the FractalForums site. Operating System: Windows, Linux.

Like Mathematica, Sage can solve a wide variety of higher-level math problems and is suitable for advanced students and researchers. You'll get the most functionality out of it if you know the Python programming language. Operating System: Windows, Linux, OS X

Used by many industrial engineers and university researchers, Scilab can perform hundreds of mathematical operations. It also comes with an API for those who want to use its capabilities to create their own applications. Operating System: Windows, Linux, OS X

If you need to work with numbers that are too big for the standard Windows calculator to handle, TTCalc is the solution. It includes most arithmetic and trigonometric functions, and it's accurate to 306 valid decimal digits. Operating System: Windows

Rated as "one of the most user-friendly solution with highly ergonomic windows display" by the World Bank/CGAP, Octopus offers a robust, secure MIS for microfinance organizations. It's available both in a free community version or a supported professional edition. Operating System: Windows

Open Source Apps: Mind Mapper

This tool is excellent for brainstorming sessions, creating diagrams, outlining the steps of a project, process management and more. The best way to see how it works is to take a look at the screenshots on the website. Operating System: OS Independent

Similar to FreeMind, XMind has one numerous awards, and it's been downloaded more than 1.7 million times. It also comes in an paid enterprise version that adds Gantt charts, privacy/sharing features, company themes and more export options. Operating System: Windows, Linux, OS X.

Funambol includes a number of mobile-related tools in one package: a data synchronization service, a device management service, client connectors and a software development kit. The site also acts as a forge hosting multiple projects built with Funambol tools. Operating System: Android, iOS, Windows Mobile, Symbian

This is an open source version of the LISP-Mobile Node implementation, which provides mobile devices greater ability to roam while remaining connected to the Internet. It was originally developed by Cisco and is now maintained by Barcelona Tech University. Operating System: Linux

QuincyKit collects information about app crashes and reports them to your server. It automatically generates reports about similar types of crashes and gives you the option of collecting user feedback. Operating System: OS X, iOS

Still in the extremely early stages of development, Tizen is the mobile OS meant to replace Meego. Development is being led by Intel, Samsung and the Linux Foundation; a first release is expected in the first quarter of 2012. Operating System: N/A

Open Source Apps: Modeling

The self-proclaimed "leading open source UML modeling tool" supports all standard UML 1.4 diagrams and comes in 10 different languages. Because it's based on Java, it runs on any platform, and it can export diagrams in six different file formats. Operating System: OS Independent

Designed as an alternative to Rational Rose and other commercial modeling tools, StarUML supports both the latest UML standards and Model Driven Architecture (MDA). It's very user-friendly and features a plug-in architecture. Operating System: Windows

The code behind Modelio has actually been in development for more than 20 years, but parent company Modeliosoft just recently released it under an open source license. It offers a UML modeler, BPMN support, Java code generator, and support for XML, HTML and Jython. Operating System: Windows, Linux

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